A variety of processes have been proposed to date as processes for selectively hydrogenating the carbon-carbon double bonds of an unsaturated, nitrile-group-containing polymer to obtain a highly saturated polymer having a high hydrogenation degree. As a general process, there is a process in which an unsaturated, nitrile-group-containing polymer in a solid form is dissolved in an organic solvent, followed by hydrogenation with hydrogen in the presence of a catalyst (Japanese Patent Application Laid-Open Nos. 117501/1984, 17103/1983, 42937/1987, 125858/1987, 181304/1987, 247706/1986, etc.).
The above process in which the unsaturated, nitrile-group-containing polymer is hydrogenated in the form of a solution does not require particularly complex steps when employed for the hydrogenation of an unsaturated polymer obtained by solution polymerization, because its hydrogenation can be effected by directly introducing a catalyst and hydrogen into a polymer solution obtained after completion of the polymerization reaction. Unsaturated, nitrile-group-containing polymers are however obtained as aqueous emulsions by emulsion polymerization in many instances. Further, saturated polymers obtained after hydrogenation are often employed as aqueous emulsions.
Hydrogenation of an unsaturated, nitrile-group-containing polymer is therefore carried out in the following manner. (1) A reagent is added to an aqueous emulsion obtained by emulsion polymerization, whereby the emulsion is broken to precipitate or coagulate the polymer. The polymer is collected by filtration and then dried to obtain the polymer in a solid form. (2) The solid polymer is dissolved in an organic solvent to provide a solution. While in the form of the solution, the polymer is subjected to hydrogenation in the presence of a catalyst and hydrogen. (4) Thereafter, the solvent is removed from the reaction mixture to obtain a hydrogenated, saturated polymer.
Further, to obtain an aqueous emulsion of the saturated polymer obtained by the hydrogenation, a special step is required for emulsification. As is envisaged from the foregoing, hydrogenation of a polymer in the form of a solution requires to go through many steps. Its practice on an industrial scale hence involves many problems.
Another process has also been proposed, in which an aqueous emulsion of an unsaturated, nitrile-group-containing polymer is added with a solvent capable of dissolving or swelling the unsaturated polymer and also with a catalyst solution, followed by hydrogenation of the polymer in an emulsified state (Japanese Patent Application Laid-Open Nos. 115303/1984 and 133219/1984; U.S. Pat. No. 3,898,208, etc.).
The known processes referred to above however use a costly rhodium (Rh) catalyst as a hydrogenation catalyst. Rh is contained at a level as low as about 0.005 ppm by weight in the earth crust and is an extremely scarce resource. To conduct hydrogenation in the presence of an Rh catalyst on an industrial scale, a serious problem therefore lies from the standpoint of economy. Incidentally, platinum (Pt) or palladium (Pd) is contained at a level as high as about 0.01 ppm by weight in the earth crust.
Further, the process for the hydrogenation of an unsaturated polymer in the form of a latex, which is disclosed in Japanese Patent Application Laid-Open No. 161415/1984, uses hydrazine or a derivative thereof as a hydrogen source. Compared with processes in which economical gaseous hydrogen is used, this process however involves an economical problem for its practice on an industrial scale.
An aqueous emulsion of a saturated (hydrogenated) nitrile-group-containing polymer is useful, for example, as an adhesive for bonding rubber with various fibrous materials by curing. A variety of processes have therefore been proposed for the preparation of such an aqueous emulsion. Of these, the following two processes may be mentioned as principal ones.
According to the first process, an unsaturated, nitrile-group-containing polymer is hydrogenated and from the resulting saturated polymer obtained in a solid form, an aqueous emulsion is prepared by the phase inversion method. Namely, the unsaturated, nitrile-group-containing polymer in the solid form is firstly dissolved in an organic solvent, followed by hydrogenation in the presence of a hydrogenation catalyst and hydrogen. The solvent is thereafter removed to obtain a saturated, nitrile-group-containing polymer in a solid form. Next, the saturated, nitrile-group-containing polymer thus obtained is dissolved in an organic solvent. An emulsifier and water are added to the organic solvent solution, and the resultant mixture is agitated into an emulsified state by a high-speed agitator. The organic solvent is then removed by the steam stripping technique or the like carefully to avoid breakage of the emulsion thus obtained, whereby an aqueous emulsion of the saturated nitrile-group-containing polymer is obtained. This phase inversion method however requires to go through many steps as described above. It is accompanied by another problem that the emulsion obtained as described above has a relatively large average droplet size and can hardly provide sufficient bonding strength when employed as an adhesive.
The second process comprises hydrogenating an unsaturated, nitrile-group-containing polymer as it is, namely, in the form of an aqueous emulsion to retain a state emulsified in water even after the hydrogenation. For example, reference may be had to the process disclosed in U.S. Pat. No. 3,898,208, in which a state emulsified in water is obtained after completion of hydrogenation. This process is however applicable to oil-insoluble conjugated diene polymer latexes. Applicable polymers are hence limited to those already crosslinked three-dimensionally before hydrogenation. This process is therefore not intended for uncross-linked polymers soluble in organic solvents. On the other hand, in the processes disclosed in Japanese Patent Application Laid-Open Nos. 115303/1984 and 133219/1981, respectively, an expensive Rh catalyst is employed as described above. These patent publications are silent about whether a state emulsified in water is retained after hydrogenation or not. These processes are therefore still insufficient as industrial processes for the preparation of an emulsion of a saturated, nitrile-group-containing polymer.